1. Field of the Invention
The present invention relates to a control method of an injection molding machine preferable to use it for molding a relatively thin molding article such as a compact disk.
2. Description of the Relevant Art
Up to now, in a case where a molding is performed by means of an injection molding machine, as disclosed in Japanese Patent Publication No. 3(1991)-39816, at the time of injection and filling, a screw was advanced by speed control to inject and fill the resin in a barrel cylinder into a mold, and thereafter, a predetermined holding pressure was applied to the resin in the mold by pressure controlling the screw. In this resin, if the screw is advanced through the speed control, the load pressure of the screw slowly rises because the resin is filled in the mold. When the screw reached a speed-pressure changing point, the screw was changed from the speed control to the pressure control and predetermined holding pressures, for example, a first holding force and a second holding force were applied successively to the resin in the mold for a predetermined set time respectively.
Meanwhile, in a case where a relatively thin molding article such as a compact disk or laser disk is molded, there is used a method in which the holding pressure is stressed and weakened according to a pulse width and a pulse interval of a pulse signal at the time of applying the holding pressure, as disclosed in Japanese Patent Laying Open No. 1(1989)-280524, since resin hardly enters the entire mold cavity, besides the resin in the cavity of the die is easily applied with stresses. That is apparently due to the fact that the screw cannot be controlled in the longitudinal direction and cannot apply a positional vibration in the longitudinal direction, since the injection cylinder having a one-side rod piston is used in the apparatus of this Jpaneses Patent.
Accordingly, there are problems in the case of existing methods wherein the resin easily enters into the entire mold cavity and the stress of the resin in the mold cavity can be decreased. However, a holding pressure becomes unstable and a molding failure such as an overpack and a burr and the like easily occurs, and the weight fluctuation of molding articles easily occurs due to the fluctuation of the amount of resin to be injected into the mold cavity when the pressure applied to the resin fluctuates due to the disturbance of the flowing state, temperature or the like of the resin.